Downsides to salt pools

[chem geek]

There's been talk about various sources of electrochemical corrosion and stray voltages and currents so I want to address some of this so that we can try and figure out what is really going on. First, I want to distinguish between different types of corrosion sources for metal in conductive solutions or exposed to moist soil or moist air:

1) "Normal" corrosion. This is where there is a single metal exposed to some oxidizer (usually oxygen in air and dissolved oxygen in water, though in pools chlorine is by far the more powerful oxidizer). Such corrosion is usually slow unless some deep pits form at which point the corrosion begins to look more like an electrolytic cell with a lower concentration of oxidizer deep in the pit and with electron flow in the metal and ion flow in the solution between the surface and the bottom of the pit.

2) "Galvanic" corrosion. I use this term to refer to when dissimilar metals are touching or connected electrically (by a wire, or a common connection to ground, etc.). In this case, the more anodic metal will corrode and will generally do so much more quickly than with corrosion of a single metal. This technique is used intentionally to protect steel pipes from corroding by connecting them electrically to zinc "sacrificial anodes". Zinc will corrode much faster than steel and when connected together, it essentially imparts a net negative potential on the steel which slows down its corrosion.

3) "Electrolytic" corrosion. I use this term to refer to a forced potential difference that is applied to two metals in a conductive solution (or moist ground, etc.). The potential difference must be DC in nature in order for electrolysis to occur and it may come from stray voltages or may be intentional (as in an electrolytic cell). The corrosion rate is high for this type of corrosion and is related to the potential difference, but it is non-linear and the rapid corrosion rate will only start to occur after the appropriate potential difference is achieved, though this is typically only a volt or two.

4) "Passivity" corrosion. I use this term to refer to corrosion that occurs when there is interference with the formation of a passivity or protective layer that normally prevents or slows down corrosion. Stainless steel is the most common example and I've discussed elsewhere in this thread how chlorides (from salt) can interfere with the formation of the passivity layer and how sulfates may accelerate this process.

A valid question is whether the SWG cell itself can be a source of DC voltages, so to discuss this let me first show what normally goes on if there is no leakage of voltage/current:

                          < e-                               < e-
              ______________________ DC Voltage from ____________________
           e- |                      Transformer/Diodes                 | ^
           v  |                                                         | e-
              |                                                         |
Negative Plate releasing electrons                        Positive Plate absorbing electrons
2H2O + 2e- --> H2(g) + 2OH-                               Cl- --> HOCl + H+ + 2e-

             Cl-, OH- >             H+ + OH- --> H2O          < HOCl, H+

Notice that what goes on is that there is a current flow of electrons in the wire between the plates and that this current is driven from a voltage source, typically stepped down AC voltage from a transformer that is then converted to DC voltage by a diode bridge or equivalent circuit. There is also an ion charge flow of negative ions moving from the left to the right and positive ions moving from the right to the left -- both of which represent a NET negative charge flow from left to right. I also show how chloride ions will diffuse from left to right since they are used up (converted to chlorine) on the right and how chlorine (HOCl) diffuses from the right to the left. What mostly goes on, however, is that the hydroxyl ion produced on the left combines with the hydrogen ion produced on the right to form water.

In the above cell, there is water flow (from the pump) and this will tend to push both the positive and negative ions (and neutral species as well) downstream possibly out of the cell and into the pipe. However, both positive and negative charges are in the flow so there is no NET charge flow and therefore no ion current (except between the plates as described above). If there were some sort of net charge flow down the pipe, then there would have to be some sort of net charge creation in the cell. This is possible, but only if there is a leak of electrons so that the circuit is complete. This is shown in the following diagram:

                                                                              < e-
                         < e-                               < e-      .........................
              ______________________ DC Voltage from _________________|__                     :
           e- |                      Transformer/Diodes                 | ^                   :
           v  |                                                         | e-                  :
              |                                                         |                     :
Negative Plate releasing electrons                        Positive Plate absorbing electrons  :
2H2O + 2e- --> H2(g) + 2OH-                               Cl- --> HOCl + H+ + 2e-             :
                                                                                              :
                                    LESS         LESS           LESS LESS                     : ^
             Cl-, OH- >              H+ + OH- --> H2O         < HOCl, H+                      : e-
                                                                                              :
                                                                                              :
                                       OH-     ^                                              :
                                        v    Fe(2+)                                           :
                                                                     e- >                     :
                                       Metal in Pool .........................................:
                                   Fe(s) --> Fe(2+) + 2e-

Notice that now there is an electron current leak (it's a positively charged leak in the sense that it wants to "receive" electrons) and this causes less positively charged hydrogen ions to be produced at the plate so when all ions are pushed down the pipe with the water flow, there is a net negative charge due to an excess of hydroxyl ions (OH-) over hydrogen ions (H+). In addition, at the metal in the pool, the metal (iron, in this example) corrodes so forms ferrous ions in the water and these are excess positive charges that migrate back toward the cell (through the floor drain and skimmer since the ions won't generally diffuse "upstream" through the returns). So there is a net negative ion current flow from the salt cell to the metal in the pool, thus completing the circuit. In essence, the electrical connection between the wire connected to the positive plate in the salt cell and the metal in the pool effectively "extends" this plate to be partly in the cell and partly in the pool. The problem is that the metal in the pool isn't made out of platinum, titanium, graphite, or another material that does not corrode, so instead of making chlorine, the metal corrodes since that reaction is much more favored. The resistance of this long flow path is obviously much higher than between the plates so the current will be small, but even a current of 2.5 milliamps would result in 1.6 ounces of metal corrosion in a year of "on" time (25 milliamps would corrode 1 pound of metal in a year).

The question becomes, "can the plate or wire on one side of the salt cell (especially the positively charged side) become connected to metal in the pool?" There is a bonding wire and it seems as if it is intentionally connected to the SWG, but I do not understand why. I can see that for most metal that is in devices that carry electricity, such as the pump, that you want to bond it to other metal in the pool so that no potential difference exists (essentially shorting out an electrolytic circuit), but you obviously can't connect the two plates in the salt cell together or else they will short out and no electrolysis will occur. You really shouldn't connect one side either, for the reasons shown above in the diagram. Perhaps the salt cell is bonded in case the electrical housing gets wet, but that seems strange to me. Does anyone know how the bonding to the SWG is done? Has anyone looked inside their SWG power box to see what the bonding wire is connected to and if there is any way for it to be electrically connected to a wire going to a plate?

If DC voltages are measured at the pool, say between the pool water and some metal or the ground, then it should be trivial to turn off the SWG (perhaps even unplug it) and see if the DC voltages remain or go away (or measurably decline). If the SWG is not the source of DC voltages, then these could be traced back to the source. It is not uncommon to find AC voltage differences around a pool due to the electrical distribution system which shunts the neutral to ground in the power system. The neutral is not completely neutral so ground voltages (and currents) are possible. However, AC voltages do not cause electrolysis and therefore will not cause corrosion. Bonding all pool metal together (as well as a grounding grid in coping) removes this potential difference by shunting the current to the bonding wire instead of inside you!

Richard

[chem geek]

A related topic to the post above is "what is the conductivity of an SWG salt pool compared to a non-salt pool?" The conductivity of 3000 ppm salt water (at 25C or 77F) is 5.7 milliSiemens per centimeter. This conductivity is approximately linear with the amount of salt so an SWG pool is approximately 3-6 times as conductive as a non-salt pool (assuming typical 500-1000 ppm TDS for a non-salt pool).

In terms of what this conductivity means for resistance, that depends on the area of the metal that is in the water and the distance between them. The formula is C=G*(L/A) where C is specific conductivity in milliSiemens per centimeter, G is conductivity which is the inverse of resistance (R), L is the distance between the plates and A is the plate area. So, G=C*(A/L) and R/L = 1/(G*L) = 1/(C*A), but converting to Ohms (from the inverse of milliSiemens) gives R/L (ohms per centimeter) = 1000/(C*A) = 1000/(5.7*1) = 175 Ohms per centimeter.

For 1 meter square area plates, we have 1000/(5.7*100*100) = 0.0175 Ohms per centimter or 1.75 Ohms per meter. So clearly the area of the exposed metal is a critical factor.

The area of a light ring or a handrail (for the portion underwater) is probably around 200 square inches or 1300 square centimeters so that becomes 1000/(5.7*1300) = 0.135 Ohms per centimeter or 4.1 Ohms per foot. For comparison, a non-salt pool has a resistance (for the same amount of exposed metal) of around [EDIT] 12 to 24 [END-EDIT] Ohms per foot. In other words, an SWG salt-pool is more conductive (has lower resistance), but even a standard non-salt pool still conducts electricity quite a bit.

One factor to consider for which I have no detailed information (yet) is how water flow affects conductivity. I would think that there would be rather high resistance to net ion charge current flowing "upstream" of water flow and rather low resistance to that current going with the water flow. So actual resistance may be lower than just calculated when taking into account water flow. If this is true, then metal objects closer to the "return-to-skimmer" and "return-to-drain" flows would be more likely to corrode. However, there are also differences in chemistry in such water from the SWG through the returns since the water is higher in chlorine level and is higher in pH (is more alkaline) so is therefore more corrosive to metal (due to higher chlorine) and may precipitate hydroxides of metal ions that may be in the water, but this is really no different in principle, and in fact is much milder, than what occurs if one adds bleach or chlorinating liquid to the pool.

One thing that doesn't make a lot of sense is the graph shown in this post where it shows that the relative corrosion rate of steel in milligrams per square decimeter went up a very small amount (not even doubling) with even factors of 10 increases in salinity. Apparently, "normal" corrosion is not nearly dependent on the conductivity of the water and probably occurs directly as a chemical reaction rather than electrolytically so is probably most dependent on the concentration of the oxidizer (oxygen in air, dissolved oxygen in water, or disinfecting chlorine). Either that, or some localized corrosion has greater localized conductivity possibly due to some corrosion products. However, if there is a potential difference over a greater distance as with galvanic corrosion or electrolytic corrosion, then corrosion should be more directly and proportionately related (approximately linearly, in the range of TDS found in pools) to the salinity and specifically the conductivity of the water.

I received some confirmation regarding the SWG electronics and what I said was essentially correct in terms of a transformer (to lower AC voltage) followed by a rectifier bridge (to convert AC to DC) plus some other circuitry designed to ensure isolation of the AC voltages. The bonding wire is a secondary safety ground protection so does not connect to the plate wires. So if there were a voltage/current leak as I described in the previous post, then it would not be intentional (i.e. it's not a faulty design).

Another possibility again related to any voltage/current electrical leakage is if this leak went to "ground" which can be assumed to be a "sink" or "source" for electrons. The following shows what would happen in this case:

                                                                              < e-
                         < e-                               < e-      .........................
              ______________________ DC Voltage from _________________|__                     :
           e- |                      Transformer/Diodes                 | ^                   :
           v  |                                                         | e-                  :
              |                                                         |                     :
Negative Plate releasing electrons                        Positive Plate absorbing electrons  :
2H2O + 2e- --> H2(g) + 2OH-                               Cl- --> HOCl + H+ + 2e-             :
                                                                                              :
                                    LESS         LESS           LESS LESS                     : ^
             Cl-, OH- >              H+ + OH- --> H2O         < HOCl, H+                      : e-
                                                                                              :
                                                                                              :
                                       OH-                                                 Ground
                                        v

In this case, notice that though there isn't direct electrolytic corrosion, there is a buildup of charge (in this example it's negative charge, but it could instead be positive charge if the opposite plate were connected to ground instead). Essentially, the pool acts like a capacitor. A voltage would be measured from the pool to the ground and to any metal which may be bonded. However, if the bonding wire is not fully connected to ground (moist soil), then the charge in the pool can build up. If metal in the pool were bonded and such bond were connected to a good ground, then the charge in the pool would likely be discharged and would not build up (though this depends on the rate of the charge buildup relative to the discharge rate).

In the above example, the voltages are not usually sufficient to cause electrolytic corrosion and there is not an electron path to the pool metal, but if the bonding wire were connected to ground, then the Ground-to-Ground would be the electron path and corrosion would be possible just as with the example where a direct wire was connected (though the resistance through ground is clearly much higher than the direct wire example). Even with lower voltages, this still lowers the activation energy for corrosion so could still speed it up, but not as much as with electrolysis (remember that the oxidation potential of chlorine far exceeds what is needed to corrode iron so it is not a question of "if", but of "how quickly").

Richard